Antistats or antistatic agents are used to dissipate electrostatic or static charge. Electrostatic charge buildup is responsible for a variety of problems in the processing and the use of many industrial products and materials. Electrostatic charging can cause materials to stick together or to repel one another. In addition, static charge buildup can cause objects to attract dirt and dust that can lead to fabrication or soiling problems and can impair product performance. Sudden electrostatic discharges from insulating objects can also be a serious problem. When flammable materials are present, a static electric discharge can serve as an ignition source, resulting in fires and/or explosions.
Electrostatic charge is a particular problem in the electronics industry, because modern electronic devices are extremely susceptible to permanent damage by electrostatic discharges. The buildup of electrostatic charge on insulating objects is especially common and problematic under conditions of low humidity and when liquids or solids move in contact with one another (tribocharging).
Static charge build-up can be controlled by increasing the electrical conductivity of a material. This can be accomplished by increasing ionic or electronic conductivity. The most common means of controlling static accumulation today is by increasing electrical conductivity through moisture adsorption. This is commonly achieved by adding moisture to the surrounding air (humidification) or by the use of hygroscopic antistatic agents, which are generally referred to as humectants because they rely on the adsorption of atmospheric moisture for their effectiveness. Most antistatic agents operate by dissipating static charge as it builds up; thus, static decay rate and surface conductivity are common measures of the effectiveness of antistatic agents. Antistatic agents can be applied to the surface (external antistatic agent) or incorporated into the bulk (internal antistatic agent) of the otherwise insulating material. Internal antistatic agents are commonly employed in polymers such as plastics.
Generally, internal antistatic agents fall into one of the following classes: (I) ones that are mixed directly into a molten polymer during melt processing; (2) ones that are mixed into a polymer solution, coated, and dried, or (3) ones that dissolve into a monomer (with or without a solvent) that is subsequently polymerized.
Antistatic agents are known and used over a broad range of chemical classes, including organic amines and amides, esters of fatty acids, organic acids, polyoxyethylene derivatives, polyhydric alcohols, metals, carbon black, semiconductors, and various organic and inorganic salts. Many are also surfactants and can be neutral or ionic in nature. Many low molecular weight, neutral antistatic agents have sufficiently high vapor pressures and thus are unsuitable for use at high temperatures (e.g., polymer melt processing) due to material losses that occur via evaporation. Many other neutral antistatic agents have insufficient thermal stability to survive polymer melt processing or other high temperature processing conditions.
Most non-metallic antistatic agents are generally humectants that rely on the adsorption and conductivity of water for charge dissipation. Thus, their effectiveness is typically diminished at low atmospheric humidity. Because many of these antistatic agents are also water-soluble, they are easily removed by exposure of the material to water (e.g., washing) and therefore are not very durable.
Metal salts of inorganic, organic, and fluoroorganic anions are also useful as antistatic agents in certain polymer compositions. Alkali metal salts are most commonly used due to cost and toxicity considerations and to the high affinity of alkali metal cations, especially lithium, for water. However, most metal salts are not compatible with polymers of moderate to low polarity, such as polypropylene, polyester, and polycarbonate. This incompatibility can result in inadequate antistatic agent performance and/or an unacceptable reduction in physical properties or transparency in a finished polymeric article. Consequently, the use of metal salts as internal antistatic agents is generally limited to highly polar and/or hydrophilic polymer matrices.